In the past, dimming circuitry for hot cathode fluorescent lamps incorporated variable frequency/variable duty cycle single ended DC power supplies. The arc supply voltage ionizes the mercury/argon vapor in the fluorescent lamp by energizing the filaments so that one filament acts as the electron emitting cathode electrode. One of the consequences of using single ended DC lamp supplies is uneven filament wear resulting in premature cathode degradation which shortens the life of the fluorescent lamp.
Applicant has found that even filament degradation and increased life for the fluorescent lamps may be realized by providing a symmetrical current through the fluorescent lamp by changing the polarity of the arc voltages applied to the electrodes so that each filament alternately acts as the anode and as the cathode. Both filaments experience similar degradation with time at all brightness levels thereby prolonging lamp life.